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    "## ARIMA\n",
    "### 平稳性\n",
    " * 平稳性就是要求经由样本时间序列所得到的拟合曲线在未来的一段时间内仍能顺着现有的形态“惯性”地延续下去\n",
    " * 平稳性要求序列的均值和方差不发生明显的变化\n",
    "\n",
    "### 严平稳与弱平稳\n",
    " * 严平稳：表示的分布不随时间的改变而改变\n",
    "     如白噪声（正态），无论怎么取，都是期望为0，方差为1\n",
    " * 弱平稳：期望与相关系数（依赖性）不变\n",
    "     未来某时刻的t的值Xt就要依赖于它的过去信息，所以需要依赖性\n",
    "\n",
    "### 差分法\n",
    " * 差分法：时间序列在t与t - 1时刻的差值"
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